I'm using two 500W titanium heaters TSH-500 from Jehmco
http://www.jehmco.com/PRODUCTS_/HARD...s/heaters.html
with their industrial controller ETC-1R on a 220 with 120g sump. The sump is on the basement floor.
I've tried Visitherm and Ebo and wasn't too sure about Won, I read some good things about these and the price was about the same as a couple of big Ebo's from a LFS. The heaters do not have thermostats (that could stick) so they have to plug into a controller. The temp in the tank stays within 1 degree.